How to read a genetic test result
A DNA report can look like a row of letters: N/N, N/HRD, N/PRCD. Each result
follows the same pattern. Once you can read it, you can tell what a result means for
the animal and for its offspring.
Two letters, one from each parent
Every result describes the two copies of one spot in the genome, one inherited from
each parent. Labs write the two copies with a slash between them. N usually stands
for the normal version, and the other letters name the variant the lab tested for.
UC Davis’s Veterinary Genetics Laboratory, for example, writes HERDA results with
HRD for the variant, so a horse’s result reads N/N, N/HRD, or HRD/HRD.
- Two normal copies (
N/N): the animal doesn’t have the variant and can’t pass it on. - One normal copy and one variant (
N/HRD): the animal can pass the variant to about half of its offspring. - Two copies of the variant (
HRD/HRD): every offspring inherits one copy.
What one copy means depends on the condition
This is where most misreadings happen. Whether one copy matters for the animal itself depends on how the condition is inherited, and the letters alone don’t tell you.
- Recessive conditions need two copies to cause the disease. An animal with one
copy is a carrier: healthy itself, but able to pass the variant on. HERDA in
Quarter Horses works this way, and UC Davis describes
N/HRDhorses as unaffected carriers. - Dominant conditions need only one copy. HYPP in Quarter Horses works this way,
so an
N/Hhorse can show signs of the disease itself. It isn’t just a carrier.
Before you read a result, check the mode of inheritance the lab gives for that test. Most lab reports and test pages state it.
Two carriers: the one-in-four rule
For a recessive condition, breeding two carriers gives, on average:
- one offspring in four affected,
- one in two carriers,
- one in four with two normal copies.
A carrier bred to a mate with two normal copies produces no affected offspring for that condition, although about half may be carriers. That’s why a carrier can stay in a breeding program: pair it only with tested-clear mates. The guide to recessive-risk pairing goes further.
”Clear” covers only what was tested
A clear result means the animal doesn’t have the specific variant the lab tested for.
It says nothing about other variants, even ones with a similar name. Progressive
retinal atrophy (PRA) in dogs is a good example. Several forms exist, and UC Davis
describes prcd as one inherited form. A dog that tests N/N for prcd won’t develop
that form, but the result says nothing about the others.
When you compare results, check the exact test or variant name, not just the condition’s common name. Two reports that both say “PRA: clear” may not have tested the same thing.
What to keep with every result
A result is only as useful as the details that come with it. Keep:
- the lab report itself, not just a note of the outcome;
- the lab’s name, and the test or variant it covered;
- the animal’s name and identifiers as the lab recorded them, such as a registration number or microchip;
- the date of the report.
These details let a buyer, a registry, or your own next breeding decision rely on the result.
Where AnimalTrace fits
AnimalTrace keeps each lab report on the animal’s record, exactly as the lab issued it. With Client Pro, breeding checks use the lab carrier results on record for both animals to show the odds for each condition both have been tested for. Using results from reports you upload yourself in those checks is planned.